1. Field of the Invention
The present invention relates to an optical fiber grating, and relates specifically to a variable optical fiber grating in which the amount of chromatic dispersion can be varied.
2. Description of the Related Art
An optical fiber Bragg grating (hereafter referred to as “FBG”) is an optical fiber component having a characteristic of forming a periodic refractive index distribution in the longitudinal direction of the optical fiber and reflecting light of a specific wavelength. The reflected wavelength λ of this FBG can be expressed by equation (1), using the grating period Λ and the effective refractive index neff.λ=2neffΛ  (1)
This FBG is used in optical multiplexer/demultiplexers, optical switches, optical filters and the like, and is an essential component for optical communication. A chirped optical fiber grating, in which the grating period or the effective refractive index varies along the longitudinal direction of the optical fiber, is one example of an FBG. Chirped optical fiber gratings are widely used in broadband filters, but their application to chromatic dispersion compensation optical fiber gratings, which compensate for the accumulation of chromatic dispersion in the optical transmission path, is of particular interest.
FIG. 8 shows an outline of the structure of a dispersion compensation optical fiber grating. In FIG. 8, reference symbol 11 denotes an optical transmission path, and reference symbol 12 denotes an optical circulator. Light, which enters a port 1 of the optical circulator 12 from the transmission path 11, is transmitted to a port 2, and enters an optical fiber grating 13. This optical fiber grating 13 comprises a core 14 and a cladding 15, and a high refractive index section is provided on the core 14 to form a grating section 16. This optical fiber grating 13 is a chirped optical fiber grating, in which the period of the high refractive index section or effective refractive index varies along the longitudinal direction thereof.
The light, which enters the optical fiber grating 13, is reflected in the grating section 16, but since the grating period varies along the longitudinal direction of the optical fiber and the reflection position differs according to the wavelength, the reflected light travels different optical path lengths, determined by its wavelength. In the case shown FIG. 8, long wavelength light is reflected at the left side of the grating section 16 which has a long grating period or high effective refractive index, and short wavelength light is reflected at the right side of the grating section 16 which has a short grating period or low effective refractive index. When this reflected light extracted from the port 3 via the optical circulator 12, a time difference occurs for each wavelength. This wavelength dependent time difference is called chromatic dispersion, and is normally expressed in units of ps/nm.
For a wavelength of 1550 nm, the single mode optical fiber generally used in optical communication has chromatic dispersion of approximately 17 ps/nm per 1 km of fiber length. If chromatic dispersion is large, this results in widening of the width of the optical pulses transmitted through the optical fiber, and hence the information cannot be transmitted accurately because each pulse of a pulse train overlaps each other. Consequently, by using a dispersion compensation optical fiber grating designed to have the exact opposite chromatic dispersion to the chromatic dispersion produced by the optical fiber which constitutes the optical transmission path, it is possible to compensate for the chromatic dispersion which has accumulated in the optical transmission path, which leads to a considerable improvement in the optical communication system.
However, the chromatic dispersion produced in the optical transmission path varies according to the type and length of the optical fiber used. Consequently, in order to achieve complete dispersion compensation, it is necessary to design individual dispersion compensation optical fiber gratings for each fiber to be compensated.
Normally, the optical fiber grating is manufactured using a phase mask, but in order to manufacture optical fiber gratings with different specifications, phase masks with different characteristics are required. These phase masks are expensive, and hence the price of dispersion compensation optical fiber gratings is accordingly high.
In addition, because the chromatic dispersion of an optical transmission path varies according to environmental variations such as variations in temperature, the required amount of dispersion compensation varies between night and day and with the seasons. Consequently, a problem of ordinary dispersion compensation optical fiber gratings is that chromatic dispersion cannot be completely compensated for under all conditions. As a result, a dispersion compensation optical fiber grating which has a variable construction in which the dispersion characteristics can be varied according to need is required.
In order to change the dispersion characteristics of the dispersion compensation optical fiber grating, it is necessary to control the longitudinal direction dependency of the reflected wavelength, and several methods of such control have been proposed. One of these is a method in which a temperature distribution is applied to the optical fiber grating, and another is a method in which a strain distribution is applied.
The method in which a temperature distribution is applied to the optical fiber grating is a method in which the reflected wavelength is varied by providing a temperature distribution along the longitudinal direction of the optical fiber grating. In this method, it is required that an accurate temperature distribution be provided along the longitudinal direction of the optical fiber, but because it is difficult to obtain the desired shape for the temperature distribution using a point heat source, a distributed heat source is necessary. A method in which gold thin film is deposited on the optical fiber grating, and the temperature of the entire optical fiber grating is controlled, has been proposed as an example of such a method.
However, deposition apparatuses are expensive, which increases the manufacturing cost, and furthermore, in this method it is necessary to perform the deposition process while accurately changing the deposited film thickness in the longitudinal direction of the optical fiber grating, which requires intricate control. Furthermore, an operation for mounting electrodes on the portions on which gold has been deposited is also required, and the complex structure and need for precise operations results in poor yield, and consequently, higher costs.
Furthermore, since the reflection center wavelength of the optical fiber grating shifts towards the long wavelength side, it is difficult to change the amount of chromatic dispersion while the operating wavelength is set to a specific wavelength, which is required for a wavelength multiplexing communication.
On the other hand, the method for applying a strain distribution to the optical fiber grating changes the reflected light by applying different strains along the longitudinal direction of the optical fiber grating. However, using this method it is also difficult to obtain a construction in which the strain varies continuously along the longitudinal direction, and as such it is difficult to obtain a large variation in chromatic dispersion.